I would suggest you do some serious research on flatheads before get involved in such a major project. Flatheads are not cheap to build, especially hopped up ones. Very few people running a 4.5" cranks. Check out the HAMB at jalopyjournal.com and post your question there. There are a lot of old time flatheaders there that can give you advise. Will save you a lot of time, money and trial and error. I've been fooling with flatheads since 1955 and these guys have forgotten more than I will ever know.